[avian]

Python 3.5 is the latest you can get on Debian Stretch (oldstable) from official repositories. Stretch is still in the Long Term Support program until around 2022. Dropping support for Python 3.5 means you can't use latest pip on Stretch anymore.

[Spiritus]

That sounds like a Debian issue though. Or are you suggesting that Python (and by extension Pip) should align their support cycle with Linux distros?